There is currently a trend towards maximizing the fuel economy benefits provided by passenger car engine oils (PCEOs). In an attempt to address this need, others have formulated PCEOs with low viscosity polyaphaolefins (PAOs), such as metallocene-catalyzed PAOs (mPAOs).
US 2009/0181872 discloses lubricating oil compositions for internal combustion engines. The examples include compositions containing low viscosity metallocene catalyzed PAO (mPAO). These compositions have kinematic viscosities at 100° C. of from 8.109 cSt to 9.053 cSt, but contain low viscosity mPAO only in amounts of up to 40 wt % of the composition. Additionally, the compositions include a viscosity index improver additive component in the amount of 4.0 mass %.
US 2011/0039743 discloses lubricating oils using a 3.9 cSt “INVENTION” fluid. For example, it discloses 0W-30 and 0W-40 passenger car motor oils, and 5W-40 heavy duty diesel engine oils, using the 3.9 cSt “INVENTION” fluid. These compositions have kinematic viscosities at 100° C. of from 10.8 cSt to 13.3 cSt, but contain the 3.9 cSt “INVENTION” fluid only in amounts of up to 48.5 wt % of the composition. Additionally, the compositions include viscosity modifier additive solution in the amount of 4.0 wt % and 9.0 wt %, depending on the viscosity grade.
WO2011125879, WO2011125880 and WO2011125881 disclose lubricant compositions for an internal combustion engine comprising; (A) a polyalphaolefin that has a kinetic viscosity at 100° C. of at most 5.5 mm2/s, a CCS viscosity at −35° C. of at most 3,000 mPa·s, and a NOACK of at most 12 mass %; and (B) a mineral oil with a viscosity index of at least 120. WO2011125879 and WO2011125881 disclose that Component (A) constitutes at least 25% of the entire composition by mass. WO2011125880 discloses that Component (A) constitutes at least 10% of the entire composition by mass. WO2011125881 also discloses that the lubricant composition comprises a polyisobutylene with a mass-average molecular weight of at least 500,000. The Tables of WO2011125879, WO2011125880 and WO2011125881 do not indicate the overall kinematic viscosities at 100° C. (KV100) of the compositions, but the compositions contain the 3.458 mm2/s mPAO only in amounts of up to 30% of the composition. Additionally, each of the compositions contain combined amounts of viscosity index improver solution and polyisobutylene solution of 7.0 mass %, including diluent.
Attempts have also been made to use conventional low viscosity polyalphaolefin base stocks (PAOs) (e.g., PAO 4 cSt, KV100) to formulate engine oil compositions. Such conventional PAOs, such as conventional PAO 4 cSt, KV100, can be produced by the use of Friedel-Craft catalysts, such as aluminum trichloride or boron trifluoride, and a protic promoter.
There remains a need, however, to provide further improvements in the fuel economy benefits of PCEOs. In order to achieve such fuel economy benefits, high quality, low viscosity PAOs can be used as the primary base stock, constituting from 60 wt % to 90 wt % of the composition, along with increased amounts of VI improvers.
In order to achieve higher efficiency PCEO formulations, high quality, low viscosity PAOs are needed. This demand for high quality PAOs has been increasing for several years, driving research in alternatives to the Friedel-Craft process. Metallocene catalyst systems are one such alternative. In the past, most of the metallocene-based focus has been on high-viscosity-index-PAOs (HVI-PAOs) and higher viscosity oils for industrial and commercial applications. Examples include U.S. Pat. No. 6,706,828, which discloses a process for producing PAOs from meso-forms of certain metallocene catalysts with methylalumoxane (MAO). Others have made various PAOs, such as polydecene, using various metallocene catalysts not typically known to produce polymers or oligomers with any specific tacticity. Examples include U.S. Pat. No. 5,688,887, U.S. Pat. No. 6,043,401, WO 03/020856, U.S. Pat. No. 5,087,788, U.S. Pat. No. 6,414,090, U.S. Pat. No. 6,414,091, U.S. Pat. No. 4,704,491, U.S. Pat. No. 6,133,209, and U.S. Pat. No. 6,713,438. ExxonMobil Chemical Company has been active in the field and has several pending patent applications on processes using various bridged and unbridged metallocene catalysts. Examples include published applications WO 2007/011832, WO 2008/010865, WO 2009/017953, and WO 2009/123800.
Recent research, however, has looked at producing low viscosity PAOs for automotive applications. A current trend in the automotive industry is toward extending oil drain intervals and improving fuel economy. This trend is driving increasingly stringent performance requirements for lubricants. New PAOs with improved properties such as high viscosity index, low pour point, high shear stability, improved wear performance, increased thermal and oxidative stability, and/or wider viscosity ranges are needed to meet these new performance requirements. New methods to produce such PAOs are also needed, US 2007/0043248 discloses a process using a metallocene catalyst for the production of low viscosity (4 to 10 cSt) PAO basestocks. This technology is attractive because the metallocene-based low viscosity PAD has excellent lubricant properties.
While low viscosity metallocene-catalyzed PAOs possess excellent properties, one disadvantage of the low viscosity metallocene-catalyzed process is that a significant amount of dimer is formed. This dimer is not useful as a lubricant basestock because it has very poor low temperature and volatility properties. Recent industry research has looked at recycling the dimer portion formed in the metallocene-catalyzed process into a subsequent oligomerization process.
U.S. Pat. No. 6,548,724 discloses a multistep process for the production of a PAO in which the first step involves polymerization of a feedstock in the presence of a bulky ligand transition metal catalyst and a subsequent step involves the oligomerization of some portion of the product of the first step in the presence of an acid catalyst. The dimer product formed by the first step of U.S. Pat. No. 6,548,724 exhibits at least 50%, and preferably more than 80%, of terminal vinylidene content. The product of the subsequent step in U.S. Pat. No. 6,548,724 is a mixture of dimers, trimers, and higher oligomers, and yield of the trimer product is at least 65%.
U.S. Pat. No. 5,284,988 discloses a multistep process for the production of a PAO in which a vinylidene dimer is first isomerized to form a tri-substituted dimer. The tri-substituted dimer is then reacted with a vinyl olefin in the presence of an acid catalyst to form a co-dimer of said tri-substituted dimer and said vinyl olefin. U.S. Pat. No. 5,284,988 shows that using the tri-substituted dimer, instead of the vinylidene dimer, as a feedstock in the subsequent oligomerization step results in a higher selectivity of said co-dimer and less formation of product having carbon numbers greater than or less than the sum of the carbon members of the vinylidene and alpha-olefin. As a result, the lubricant may be tailored to a specific viscosity at high yields, which is highly desirable due to lubricant industry trends and demands. The U.S. Pat. No. 5,284,988 process, however, requires the additional step of isomerization to get the tri-substituted dimer. Additionally, the reaction rates disclosed in U.S. Pat. No. 5,284,988 are very slow, requiring 2-20 days to prepare the initial vinylidene dimer.
An additional example of a process involving the recycle of a dimer product is provided in US 2008/0146469 which discloses an intermediate comprised primarily of vinylidene.